Lubricating oil composition for internal combustion engines

ABSTRACT

To provide a lubricating oil composition for internal combustion engines, which can realize low friction in both internal combustion engines equipped with a DLC and internal combustion engines not equipped with a DLC. [Solution] a lubricating oil composition for internal combustion engines, which is characterized in that the lubricating oil composition for internal combustion engines is a composition obtained by blending a molybdenum dithiocarbamate (MoDTC) and a polyalkylene glycol (PAG) in a base oil, the weight average molecular weight of the polyalkylene glycol is 2750-4500 and the content of the polyalkylene glycol is not less than 0.05 mass % and less than 10 mass % relative to the total mass of the composition.

FIELD OF THE INVENTION

The present invention relates to a lubricating oil composition forinternal combustion engines, which contains prescribed components.

BACKGROUND OF THE INVENTION

Many lubricating oil compositions have been proposed in the past. Forexample, WO2005014763 proposes a lubricating oil that contains alubricant base oil and a sulfur-containing molybdenum complex.Meanwhile, “Wear Analysis of DLC Coating in Oil Containing Mo-DTC”,Takatoshi Shinyoshi, Yoshio Fuwa, Yoshinori Ozaki, JSAE 20077103 SAE2007-01-1969 proposes a case in which wear of a film is facilitated whenMoDTC is combined with a hydrogen-containing DLC film (a-C:H).

In the case of the lubricating oil composition according toWO2005014763, however, evaluations are conducted using a real engine.Friction reduction performance evaluations are conducted on systems inwhich both DLC contacting faces and non-DLC contacting faces not havinga DLC film, such as ordinary steel materials for engines, aresimultaneously lubricated, but if a steel material lubrication effect isgreater than a DLC contacting face lubrication effect, it is not clearwhether this is suitable for DLC films. In addition, a means for solvingthe problem in “Wear Analysis of DLC Coating in Oil Containing Mo-DTC”,Takatoshi Shinyoshi, Yoshio Fuwa, Yoshinori Ozaki, JSAE 20077103 SAE2007-01-1969 is not given.

With these circumstances in mind, the purpose of the present inventionis to provide a lubricating oil composition for internal combustionengines, which can realize low friction in both internal combustionengines equipped with DLC and internal combustion engines not equippedwith DLC.

SUMMARY OF THE INVENTION

As a result of diligent research carried out in order to achieve thispurpose, the inventors of the present invention found that by blending apolyalkylene glycol (PAG) and a specific organic molybdenum compound asadditional components in a lubricating oil composition for internalcombustion engines, specifying the molecular weight of the PAG andspecifying the blending quantity of the PAG, it was possible to realizelow friction in internal combustion engines regardless of the presenceor absence of DLC, and thereby completed the present invention.

That is, the present invention is a lubricating oil composition forinternal combustion engines, which is characterized in that thelubricating oil composition for internal combustion engines is acomposition obtained by blending a molybdenum dithiocarbamate (MoDTC)and a polyalkylene glycol (PAG) in abase oil, the weight averagemolecular weight of the polyalkylene glycol is 2750-4500, and thecontent of the polyalkylene glycol is not less than 0.005 mass % andless than 10 mass % relative to the total mass of the composition.

Furthermore, the lubricating oil composition for internal combustionengines of the present invention may be a lubricating oil compositionfor internal combustion engines that can be used in both internalcombustion engines equipped with DLC and internal combustion engines notequipped with DLC (conventional internal combustion engines).

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, it is possible to provide alubricating oil composition for internal combustion engines, which canbe used in both internal combustion engines equipped with DLC andinternal combustion engines not equipped with DLC.

Explanations will now be given of the constitution, properties andintended uses of lubricating oil compositions for internal combustionengines according to the present aspect.

In the lubricating oil compositions for internal combustion enginesaccording to the present aspect, a molybdenum dithiocarbamate (MoDTC)and a polyalkylene glycol (PAG) are blended in abase oil, and otheradditives may, if necessary, also be blended in the base oil. A detailedexplanation will now be given of the lubricating oil compositions forinternal combustion engines according to the present aspect, but thepresent invention is in no way limited to these compositions.

The base oil used in the present aspect is not particularly limited, andmineral oils, synthetic oils, vegetable and animal oils and mixturesthereof used in conventional lubricating oils and grease compositionscan be used as appropriate. Specific examples thereof include base oilsof groups 1 to 5 in the API (American Petroleum Institute) base oilcategories. Here, API base oil categories are broad classifications ofbase oil materials defined by the American Petroleum Institute in orderto prepare guidelines for lubricant base oils. In order to achieveexcellent oxidation stability, a base oil belonging to group 3 ispreferred.

The kinematic viscosity of the base oil is not particularly limited, butfrom practical perspectives such as preventing wear and achieving fuelefficiency, the kinematic viscosity at 100° C. is preferably 2-32 mm²/s,and more preferably 3-8 mm²/s.

The viscosity index of the base oil is not particularly limited, butfrom practical perspectives such as preventing wear and achieving fuelefficiency, the viscosity index is preferably 10-200, and morepreferably 100-200.

The molybdenum dithiocarbamate (MoDTC) used in the present aspect canbe, for example, a molybdenum dialkyldithiocarbamate represented byformula (1) below.

In formula 1, R¹ to R⁴ each denote an alkyl group, and X¹ to X⁴ eachdenote an oxygen atom or sulfur atom.

The alkyl groups R¹, R², R³ and R⁴ contained in the molybdenumdialkyldithiocarbamate represented by formula (1) are each independentlya lipophilic group having 2-30 carbon atoms, and it is preferable for atleast one of these four lipophilic groups to be a secondary lipophilicgroup.

Here, the molybdenum dithiocarbamate (MoDTC) used in the present aspectis preferably a molybdenum dithiocarbamate represented by formula (2)below.

In formula (2), R¹ to R⁴ each denote an alkyl group.

The content of the molybdenum dithiocarbamate used in the present aspectis not particularly limited, but is preferably 50-1500 ppm in terms ofmolybdenum content relative to the total mass of lubricating oilcomposition.

The polyalkylene glycol (PAG) is a compound in which a plurality ofalkylene glycols are polymerized, and is represented by the generalformula HO—(C_(n)H_(m)O)_(s)—H or the general formulaHO—(C_(n)H_(m)O)_(s)—OH, but is not particularly limited. In order touse a material having low oil solubility, it is preferable to use one ormore compounds selected from among polyethylene glycol, polypropyleneglycol and polybutylene glycol. From the perspective of compatibilitywith the base oil, polypropylene glycol and polybutylene glycol are morepreferred.

In addition, the weight average molecular weight of the polyalkyleneglycol according to the present aspect is 2750-4500, and preferably3000-4000. By setting the weight average molecular weight of thepolyalkylene glycol to fall within such a range, compatibility with thebase oil is improved and the coefficient of friction can be lowered.

Furthermore, the polyalkylene glycol according to the present aspect iscontained at a quantity of not less than 0.05 mass % and less than 10mass %, preferably 0.5-8.0 mass %, and more preferably 1.0-5.0 mass %,relative to the total mass of the lubricating oil composition. Bysetting the content of the polyalkylene glycol to fall within such arange, compatibility with the base oil is improved and the lubricatingproperties of the polyalkylene glycol can be exhibited.

One or more of a variety of additives, such as ash-free dispersingagents, wear prevention agents, extreme pressure additives, metal-basedcleaning agents, antioxidants, viscosity index-improving agents,friction modifiers, rust inhibitors, non-ionic surfactants,demulsifiers, metal deactivating agents and anti-foaming agents, can beblended as optional components in the lubricating oil compositionsaccording to the present aspect.

The HTHS viscosity of the lubricating oil composition of the presentinvention at 150° C. and 10⁶ s¹ is preferably 3.5 mPa·s or less, morepreferably 3.0 mPa·s or less, further preferably 2.8 mPa·s or less, andparticularly preferably 2.7 mPa·s or less. In addition, this HTHSviscosity is preferably 1.4 mPa·s or more, more preferably 2.0 mPa·s ormore, further preferably 2.3 mPa·s or more, particularly preferably 2.5mPa·s or more, and most preferably 2.6 mPa·s or more. Moreover, thishigh temperature high shear viscosity is a numerical value determinedusing the test method disclosed in ASTM D5481.

The scope of use of the lubricating oil compositions according to thepresent aspect is not particularly limited as long as the compositionsare used in internal combustion engines.

In particular, according to the lubricating oil compositions accordingto the present aspect, by setting the weight average molecular weightand content of the polyalkylene glycol to fall within specified rangesin the composition, which is obtained by blending a molybdenumdithiocarbamate and the polyalkylene glycol in abase oil, it is possibleto reduce friction on surfaces not equipped with DLC as well assignificantly reducing friction on surfaces equipped with DLC, and it istherefore possible to realize low friction regardless of the presence orabsence of DLC. As a result, the lubricating oil composition accordingto the present aspect can be used in both internal combustion enginesequipped with DLC and internal combustion engines not equipped with DLC(that is, can be used as a lubricating oil composition for internalcombustion engines equipped with DLC and internal combustion engines notequipped with DLC).

Moreover, an internal combustion engine equipped with DLC is an internalcombustion engine in which all, or at least some, surfaces that arefriction surfaces are coated with DLC. In addition, DLC (diamond-likecarbon) generally means an amorphous substance constituted mainly fromelemental carbon, and in which the bonding between carbon atomscomprises both a diamond structure (SP3 bonds) and graphite bonds (SP2bonds). Specifically, these include a-C (amorphous carbon) comprisingonly elemental carbon, hydrogen-containing a-C:H (hydrogenated amorphouscarbon) and MeC, which partially contains a metal element such astitanium (Ti) or molybdenum (Mo). Furthermore, the coefficient offriction tends to increase as the content of hydrogen in DLC increases,but it is possible to select an arbitrary hydrogen content, such as 10atom % or less, 5 atom % or less, or 0.5 atom % or less.

EXAMPLES

The present invention will now be explained in greater detail throughthe use of working examples and comparative examples, but is not limitedto these examples.

Preparation of Lubricating Oil Composition

The raw materials used in the working examples are as follows.

Base Oil

A GTL (gas-to-liquid) base oil synthesized by the Fischer-Tropschprocess, which belongs to group 3, has a kinematic viscosity at 100° C.of 7.58 mm²/s, and has a viscosity index of 141.

Additives

PAG

PEG#400 (manufactured by NOF Corporation, PEG, weight average molecularweight 100)

D-250 (manufactured by NOF Corporation, PPG, OH groups at bothterminals, weight average molecular weight 250)

D-1000 (manufactured by NOF Corporation, PPG, OH groups at bothterminals, weight average molecular weight 1000)

D-2000 (manufactured by NOF Corporation, PPG, OH groups at bothterminals, weight average molecular weight 2000)

D-4000 (manufactured by NOF Corporation, PPG, OH groups at bothterminals, weight average molecular weight 4000)

MB7 (manufactured by NOF Corporation, PPG, OH group at one terminal,weight average molecular weight 700)

MB700 (manufactured by NOF Corporation, PPG, OH group at one terminal,weight average molecular weight 3000)

50 MB-2 (manufactured by NOF Corporation, PPG-PEG, weight averagemolecular weight 200)

MoDTC

Sakuralube-165 (manufactured by ADEKA, molybdenum content 4.5 mass %,sulfur content 5.0 mass %)

1. Viscosity Index Improving Agent

Polymethacrylate-Based Viscosity Index Improving Agent

Packaged Additives

GF-5DI (compositional details: metal cleaning agent, ash-free dispersingagent, zinc dithiophosphate, rust inhibitor, corrosion prevention agent,antioxidant, ash-free friction modifier, and the like).

Lubricating oil compositions were obtained by blending and formulatingthe raw materials mentioned above at the proportions (mass percentages)shown in the Tables.

Tests

Friction Test

The lubricating oil compositions of the working examples and comparativeexamples were applied to sliding surfaces consisting of a DLC-coatedsliding member, which was obtained by coating with a diamond-like carbonhaving a hydrogen content of 0.5 atom %, and a sliding member consistingof a SUJ2 material, and a friction test was carried out. Using acylinder-on-disk SRV friction tester (ASTMD6425), the coefficient offriction was measured. The conditions are as follows: Temperature: 80°C., frequency: 50 Hz, load: 100 N. A similar friction test was carriedout by applying the lubricating oil compositions of the working examplesand comparative examples to sliding surfaces consisting of a SUJ2sliding member not coated with a DLC and a sliding member consisting ofa SUJ2 material. The results are shown in the tables.

As shown in the tables, the present invention achieves afriction-reducing effect on both metal contacting faces if a DLC film ispresent. Because a low coefficient of friction reduces the amount offrictional heat generated, it is possible to suppress softening of asurface caused by carbonization (an increase in the amount of SP2bonding) caused by friction in the case of a DLC surface and improve thewear resistance of the DLC, and a reduction in frictional resistanceleads to a reduction in stress within a DLC film and between a DLC filmand the member therebelow, and lessens the problem of a DLC coating filmdetaching.

In this way, the present invention contributes to fuel efficiency andmaintenance of a favorable state in a DLC lubrication system.

According to the present invention, in the case of metal surfaces also,the organic molybdenum compound exhibits functionality, a reduction infriction occurs and fuel efficiency is realized even in a non-DLClubricating system.

TABLE 1 comparative Examples 1-8 and working Examples 1-2 Comp CompeComp Comp Comp Comp Comp Comp Working Working Ex 1 Ex 2 Ex 3 Ex 4 Ex 5Ex 6 Ex 7 Ex 8 Ex. 1 Ex 2 Base oil Group 3 base oil 78.35 73.35 73.3573.35 68.35 77.35 73.35 68.35 77.35 73.35 Additives DI agent 10 10 10 1010 10 10 10 10 10 Polymethacrylate-based 10 10 10 10 10 10 10 10 10 10viscosity index improving agent Mo-containing friction 1.65 1.65 1.651.65 1.65 1.65 1.65 1.65 1.65 1.65 modifier - MoDTC Mo content (ppm) 700700 700 700 700 700 700 700 700 700 PAG PEG 400 — 5 — — — — — — — —D-250 — — 5 — — — — — — — D-1000 — — — 5 10 — — — — — D-2000 — — — — — 15 10 — — D-4000 — — — — — — — — 1 5 Total 100 100 100 100 100 100 100100 100 100 Cylinder Coefficient of friction, 0.096 0.121 0.099 0.091 No0.074 0.073 No 0.061 0.047 on disk steel on DLC dissolution dissolutionfriction Coefficient of friction, 0.065 — — — 0.052 0.049 0.05 0.05steel on steel

TABLE 2 comparative Examples 9-16 and working Example 3 Comp Comp CompWorking Comp Comp Comp Comp Comp Ex 9 Ex 10 Ex 11 Ex 3 Ex 12 Ex 13 Ex 14Ex 15 Ex 16 Base oil Group 3 base oil 68.35 77.35 73.35 73.35 77.3573.35 68.35 80 75 Additives DI agent 10 10 10 10 10 10 10 10 10Polymethacrylate-based 10 10 10 10 10 10 10 10 10 viscosity indeximproving agent Mo-containing friction 1.65 1.65 1.65 1.65 1.65 1.651.65 0 0 modifier - MoDTC Mo content (ppm) 700 700 700 700 700 700 700 00 PAG D-4000 10 — — — — — — — 5 MB 7 — 1 5 — — — — — — MB 700 — — — 5 —— — — — 50MB-2 — — — — 1 5 10 — — Total 100 100 100 100 100 100 100 100100 Cylinder Coefficient of friction, No 0.074 0.074 0.064 0.09 0.0960.095 — 0.1 on disk steel on DLC dissolution friction Coefficient offriction, 0.052 0.051 0.06 — 0.053 — 0.081 — steel on steel

1. A lubricating oil composition for internal combustion engines, whichis characterized in that the lubricating oil composition for internalcombustion engines is a composition obtained by blending a molybdenumdithiocarbamate (MoDTC) and a polyalkylene glycol (PAG) in a base oil,the weight average molecular weight of the polyalkylene glycol is2750-4500 and the content of the polyalkylene glycol is not less than0.05 mass % and less than 10 mass % relative to the total mass of thecomposition.
 2. The lubricating oil composition for internal combustionengines according to claim 1, which can be used in both internalcombustion engines equipped with a DLC and internal combustion enginesnot equipped with a DLC.
 3. The lubricating oil composition for internalcombustion engines according to claim 1, wherein the polyalkylene glycolis polypropylene glycol (PPG).